Photomultiplier tubes are well known in the art. Such a tube includes a housing with a window through which light is admitted. A photocathode is combined with the tube window to provide an incident surface operative to convert incident photons to electrons. The electrons enter an electric field which moves the electrons to a target responsive to the electrons to generate a pulse.
Although a variety of targets have been used in photomultiplier tubes, gain has been limited to 10 with a single stage tube. Moreover, all targets have been quite small compared to the area of the photocathode. Thus, the distance between the target and the photocathode has been necessarily been large and an electric field has been necessary to focus the electrons on the target. The result has been that photomultiplier tubes are larger and heavier.
The photocathode for a photomultiplier tube comprises a transparent substrate such as glass, with a film of a material such as cesium, potassium, aluminum or antimony or various mixtures, thereof as is well understood in the art. The film has to be uniform to permit faithful transfer of an incident image. The film is produced by evaporation from, for example, antimony leads.
There are two common methods for producing the film. The first is called the "transfer" method where the tube, absent the photocathode is maintained in a vacuum with the glass substrate positioned beside the tube in the vacuum. Antimony beads are heated to cause the film to form and the completed photocathode is cemented in place over the target without breaking the vacuum.
The Second method is the "in place" method where the glass substrate is in position within the tube and antimony beads, near the target, are heated to provide the required film. The large separation required in prior art tubes between the photocathode and the target permits the "straight line" evaporation required to produce a uniform film.